As the use of computer and telecommunication networks and related electronic systems expands to meet the needs of the 21st century, it is imperative that the highest quality be achieved in the transmission of data and voice information signals over ever-increasing distances. The ability to transmit such information at the highest possible rate and with a minimum number of errors are two critically important features of any high quality analog or digital signal transmission system.
One method of transmitting these signals is by using an individually-twisted pair of electrical conductors such as insulated copper wires. These wires are typically coated with a plastic insulating material by an extrusion process. Although these conductors have been in use for quite some time, especially in the telephone industry, asymmetrical imperfections such as ovality of the surrounding insulating material, out-of-roundness or eccentricity of the wire cross-section, and lack of perfect centering of the wire within the insulation tend to limit their ability to transmit data without an insignificant amount of error.
These imperfections are essentially unavoidable during fabrication of the individual insulated wires due to a number of factors, including necessary clearances in the extrusion tools, tool wear, gravitational forces, unequal flow of the insulating compound around the wire during extrusion, and the dragging of hot insulation against water dams and surfaces in the insulation quenching trough. As the insulation cools around the conductive portion by passing through a quenching trough immediately after extrusion, the newly insulated wire then exit the water trough where it air drys and is taken up on reels. During this process, the insulated wires rotate first in one direction and then the other due to the action of the roller guides, sheaves and traverse mechanism. This causes the orientation of the imperfections heretofore described to rotate and oscillate as the wire is transported from pay-out to take-up reels in the fabrication process, so that the imperfections do not remain in a fixed plane.
Once insulated, a conventional method for pairing two insulated wires together is by twisting them together with a double twist pairing machine. During this process, the wires receive two "lay twists," or two complete rotations about a common axis, per revolution of the machine. In addition, each individual wire is twisted two turns about its own axis per revolution of the machine in the same direction as the pair lay twists, and this is commonly referred to as "back-twist." Thus, using conventional double twist pairing, back-twist is imparted to each wire at a rate of one twist per lay twist. Upon pairing, this combination of off-center conductors, out of roundness of insulation, etc., and back-twist generally creates periodic changes in the spacing between the conductors along the length of the twisted pair.
As a result of the aforementioned asymmetrical imperfections, rotations, and changes in the spacing between conductors, a variety of transmission problems can arise. These include signal reflections (i.e., structural return loss), distortion, and loss of power. Variations in the electrical impedance of the paired wires caused by the changes in the conductor spacing give rise to signal reflections. Due to their periodic nature, these reflected signals add in phase at a specific frequency rather than randomly, thereby causing excessive loss and distortion to the transmitted signal at this frequency. This typically causes increased distortion in the amplitude and phase of the transmitted signal, leading to a reduction in the signal-to-noise ratio. This degradation of the signal shortens the distance that a signal can be transmitted along the twisted pair without error and limits the maximum frequency that can be supported.
If the two insulated wires are paired together on a pairing machine that imparts no back-twist, the periodic spacing between conductors changes from minimum to maximum at a very rapid rate of one cycle per each turn of the pair. This short distance is usually only a small fraction of the wavelength of the highest frequency transmitted on the wire pairs, thus generally making the impedance variations transparent. As a result, the advancing signal travelling down the wire pair sees only the average impedance, which possesses minimal variability in comparison to the relatively high variability in impedance experienced with cable pairs that possess the normally imparted back-twist. However, single twist pairing machines which impart no back-twist are slower than conventional double twist machines. It is generally more difficult to control the wire tension in single twist pairing machines as well. These problems can raise production costs to unacceptably high levels.
After these wires have been twisted together into cable pairs, there are various methods in the art for arranging and configuring twisted wire cable pairs into a high performance data or voice transmission cable. Such cables typically contain several pairs of twisted conductors enclosed by a plastic jacket. The most popular method is to rotate several pairs together in a process known as cabling or stranding. Once this "core" has been formed, a plastic jacket is extruded over the formed core.
Another well-known method for fabricating such a cable is by a technique known as "full pressure" extrusion. In this method, a tapered tip is shaped to receive the coupled cable pairs in one end. As the cable pairs move through this tip, the tip constricts, forcing the cable pairs into individual channels that at the end of the tip are configured along with the die for the particular form the final cable will take. For instance, four cable pairs aligned side-by-side through an oval tip and associated die will form a flat cable, while four cable pairs arranged in a circular configuration through a circular tip and round die will form a round cable.
During the full pressure extrusion process, the tip is partially placed into a die so that a gap forms between the outer surface of the tip and the inner surface of the die. This gap narrows as the die and the tip taper to the desired final cable size and shape. As the cable pairs feed through the rear of the tip, heat softened cable jacketing compound feeds under pressure into the gap between the tip and die, extruding the material out of the exit at the tapered end of the die, which is known as the die face. In the full pressure extrusion process, the tip extends only partially into the die so that when the jacketing compound extrudes through the gap to meet the cable pairs, the heat softened jacketing compound forms not only the outside shape of the cable, but may encapsulate and isolate each of the individual pairs as well.
Another well-known method for forming high-quality cable is by "semi-tubed," "semi-sleeved," or "semi-pressure" extrusion. The difference between this method and the full pressure method is that, under the semi-pressure technique, the tip extends into the die towards the die exit. This has the effect of forcing most of the extruded jacketing compound to form more loosely around the cable core, keeping the majority of the compound around the perimeter of the cable that it forms. However, depending on tip and die settings, at times the compound will begin to settle into the intersities of the cabled core, resulting in undesired jacket compound fill.
In a jacketed cable, there exists a critical area around each of the individual cable pairs in which it is ideal to maintain well defined boundaries between materials of different dielectric constants. Since air is the ideal dielectric material, it is useful to maximize the amount of air space about the pair. This is typically achieved by controlling the jacket compound filling process to create as uniform an inner surface as possible. If this process is not controlled precisely enough to provide well defined boundaries between different dielectric materials, or if excessive pressure around the cable pair distorts the geometric lay-up (i.e., twisting pattern) of the pair, increased electrical alterations can result. Under the full and semi-pressure extrusion techniques, excessive jacket compound that forms around the individual cable pairs provide the cable with a high cross-sectional strength, but tends to distort the geometric lay-up of the pairs and to alter the air dielectric about them, resulting in unacceptable electrical alterations. Another disadvantage of excessive compound fill is that, since an outer jacket is formed around each of the cable pairs, stripping the jacket from the cable in the field requires each cable pair be individually stripped of jacketing compound. In modern day applications, when increased demands are being placed on data and voice transmission systems to deliver electrical signals at the highest possible rate and with a minimum number of errors, such limitations are a substantial roadblock to achieving these goals.